Drain the water from the block and radiator and refill with a mix of Shout! (from local grocer) and water. The Shout is an excellent de-greaser and doesn't foam -- works quite well and run it thru for a few minutes, then drain, then repeat with plain water and drain again. Use MB coolant as MikeV recommended -- might as well since you've come this far.
Make sure you fill entire block by removing the 14mm bolt next to the oil dipstick (there's 2 of them side by side -- I use the one closest to cabin). When coolant starts to overflow from this hole, screw bolt back in and finish filling radiator.
Good luck and nice work!
'91 300E, 206k miles